How I Screencast on Ubuntu
I’ve been screencasting a while now, mostly about Darktable. And from time to time people ask me how I do it, and what software and hardware I use. So here goes nothing…
Since my main topic is Darktable (a free software photography application), it means my target audience is primarily photographers who use free software. Which means my video’s should be easy to view on a random Linux/BSD desktops. Considering the only video/audio codec that is available on most newly installed Linux desktop are Theora and Vorbis respectively, these were going to be my primary publishing formats. The fact that Theora and Vorbis have been the longest supported format for HTML5 Video is a big plus too (since Firefox 3.6 if I recall correctly). And I surely didn’t want to motivate/require anybody to install Flash to view my video’s.
Another point of concern was audio quality. In general when watching other peoples screencasts, the often poor audio quality was for me the biggest annoyance. Especially for longer video’s where I don’t want to listen to 20 minutes of someone talking through static noise. So I went a bit overboard with this and bought an M-Audio FastTrack MkII (which is plain USB Audio, no special driver required) and a RØDE NT1a Kit which I later on mounted to a RØDE PSA1 Studio Arm.
Which brings me to my choice of recording application. I can’t say I tried them all, but recording with ffmpeg seemed to slow down my machine too much. So I settled on recordmydesktop and more particularly the gtk-recordmydesktop frontend. After some experimenting I found recording just a part of my screen (1920×1200) to be a nuisance. So I settled on doing all screencasts on my laptop recording fullscreen (1280×800). The recordmydesktop application defaults to recording 15 frames per second, which seems to be fine for my purposes. It defaults to recording audio at a 22050 Hz sampling rate, with me being a sucker for audio quality I changed to that 48000 Hz, which is commonly used on DVDs and other professional audio applications. One of recordmydesktop’s potential disadvantages is that it only encodes it’s capture to Ogg/Theora/Vorbis (.ogv) which luckily for me really isn’t an issue at all. I do max out the encoding quality to 100% for both audio and video.
When publishing my screencasts on the web I just use the HTML5 video support of modern browsers. I use the .ogv file produced by recordmydesktop directly, I don’t re-encode to reduce the bitrate or anything, as the bitrate is already acceptable to begin with, and I don’t want to degrade the quality any more than I have to. While in the past I only provided the .ogv, I recently also caved in providing an .mp4 (H264/AAC) fallback video to be able to support the ever increasing ubiquity of tablets, and secondarily to be able to support browsers like Safari which don’t support free media formats like Ogg/Theora/Vorbis out of the box. So now I’m using ffmpeg to transcode my video, however there are a couple of concerns here. My original recordings were recorded at a resolution of 1280×800, while most tablets (and most importantly The Original iPad), only supports video at a resolution of 1280×720 (H264 Level 3.1), so it would likely choke on it. That said, in many cases it’s not very useful to have 1280×800 on most tablets anyway as 1024×768 is a common resolution for 10″ tablets. So I settled on resizing my screencasts to 1024×640 (which also reduced the bitrate a bit, in the process making it more suitable for mobile viewing). Initially I tried to do the audio using the MP3 audio codec, however iPads seem to dislike that, while Android tablets handled it just fine. So I had to go with AAC and while Ubuntu’s ffmpeg isn’t built with FAAC support, it does however have ffmpeg’s builtin AAC encoder called libvo_aacenc, which isn’t as good as FAAC, but it had to do. So in the end my conversion commandline for ffmpeg is this:
avconv -i input.ogv -sws_flags bicubic -s 1024x640 -vcodec libx264 -coder 1 \ -flags +loop -cmp +chroma -partitions +parti8x8+parti4x4+partp8x8+partb8x8 \ -me_method umh -subq 8 -me_range 16 -g 250 -keyint_min 25 -sc_threshold 40 \ -i_qfactor 0.71 -b_strategy 2 -qcomp 0.6 -qmin 10 -qmax 51 -qdiff 4 -bf 3 \ -refs 5 -directpred 3 -trellis 1 -flags2 +bpyramid+mixed_refs+wpred+dct8x8+fastpskip \ -wpredp 2 -rc_lookahead 50 -coder 0 -bf 0 -refs 1 -flags2 -wpred-dct8x8 \ -level 30 -maxrate 10000000 -bufsize 10000000 -wpredp 0 -b 1200k \ -acodec libvo_aacenc -ac 1 -ar 48000 -ab 128k output.mp4